A technique for detecting the presence or absence of excessive traffic has been described in Patent Document 1. A device described in Patent Document 1 holds a threshold value for determining whether flow reaches a flow rate greater than or equal to a predetermined bandwidth, and makes a comparison between the threshold value and the flow rate to thereby detect an excessive flow.
There is known a system in which a controller performs control on each of switches that belong to a network to thereby control a communication path or the like for each flow. An example of such a system is a system to which OpenFlow is applied. The OpenFlow is a protocol for allowing the controller to control each switch. Specifications of the OpenFlow have been described in Non-patent Document 1.